Due to the progress in different technological fields and the development in the computer industry, many light and compact products, such as the notebook computers, have become the main stream in the market. For such light and compact electronic products, their system stability and product performance or even service life are usually affected by the heat dissipation performance thereof. Taking a computer system as an example, to enable quick dissipation of the heat energy produced by the computer system during operation thereof, a fan is usually mounted to the computer system as a heat dissipation device thereof, so that the computer system can maintain normal operation in an environment having a proper working temperature.
Generally speaking, the fan used with a computer system for heat dissipation is usually driven by a brushless direct-current (DC) motor. As shown in FIG. 1, the currently available DC motor has a driving circuit mainly including a control integrated circuit (IC) 10, which transmits a pulse-width modulation (PWM) signal to a motor driving unit 12, so as to drive the motor to operate. However, when the p-type metal-oxide-semiconductor field-effect (PMOS) transistor in the motor driving unit 12 has an input power supply that is a low voltage, the voltage between the gate and the source (i.e. the so-called voltage Vgs) of the PMOS transistor would be relatively small, rendering the PMOS transistor in a not fully turned-on state. As a result, the PMOS transistor would have an increased internal resistance (i.e. an increased turn-on resistance) Rds, and accordingly, an increased turn-on power. Since the turn-on resistance (Rds) of the PMOS transistor has relation to the junction temperature (Tj) thereof, the increased turn-on resistance (Rds) also results in increased case temperature (Tc) and junction temperature (Tj) of the PMOS transistor. Therefore, the PMOS transistor in the conventional motor driving circuit has the problem of insufficiently derated junction temperature (Tj).
On the other hand, when the input voltage is a high voltage, the voltage between the gate and the source (Vgs) of the PMOS transistor is relatively large and tends to exceed the specification value therefor or have the problem of insufficiently derated voltage (Vgs).
Further, the control IC 10 usually has an operating voltage and an output signal (i.e. the PWM signal) both being high-level voltage of 5V, which is mainly used as the turn-on voltage between the gate and the source of the n-type metal-oxide-semiconductor field-effect (NMOS) transistor in the motor driving unit 12 (i.e. the turn-on voltage Vgs of the NMOS transistor). However, this high-level voltage of 5V is relatively small, rendering the NMOS transistor in a not fully turned-on state. As a result, the NMOS transistor would have an increased turn-on resistance (Rds), and accordingly, an increased turn-on power. Since the turn-on resistance (Rds) of the NMOS transistor has relation to the junction temperature (Tj) thereof, the increased turn-on resistance (Rds) also results in increased case temperature (Tc) and junction temperature (Tj) of the NMOS transistor. Therefore, the NMOS transistor in the conventional motor driving circuit tends to have the problem of insufficiently derated junction temperature (Tj). Due to these reasons, the NMOS transistor and the PMOS transistor in the conventional motor driving unit 12 always have poor heat dissipation ability that could not be effectively upgraded up to now.
In brief, the conventional fan motor driving circuit has the following disadvantages: (1) poor heat dissipation performance; (2) the voltage (Vgs) of the PMOS transistor could not be stabilized and therefore tends to exceed the specification of the turn-on voltage of the PMOS transistor and cause insufficiently derated junction temperature (Tj) of the PMOS transistor; and (3) the turn-on voltage (Vgs) of the NMOS transistor could not be effectively boosted to result in insufficiently derated junction temperature (Tj) of the NMOS transistor.
It is therefore tried by the inventor to work out a way for overcoming the problems and disadvantages in the conventional fan motor driving circuit.